Inhibitory antibodies directed against the plasminogen activator produced by a line of SV40 virus transformed hamster cells distinguish between plasminogen activators produced by different hamster tissues. These antibodies do not inhibit plasminogen activation by plasminogen activators from several lines of hamster cells transformed by other agents demonstrating that immunologically distinguishable forms (isozymes) of plasminogen activators are produced by transformed cells from the same species. The research proposed in this application will: (1) Define the number of immunologically distinguishable forms (isozymes) of plasminogen activators produced by normal hamster tissues; (2) Determine whether the transforming agent, the tissue of origin, or other factors are responsible for the production of specific isozymes by malignantly transformed hamster cells; (3) Evaluate the possibility that antibodies directed against isozymes of plasminogen activators may allow us to detect alterations in the levels of these enzymes in the sera of tumor-bearing animals.